Despite the highly developed technology for controlling the average film thickness of blown films produced by the blown film extrusion process, one cannot avoid thick and thin points that are within the range of tolerance in the blown film and that overlap on the reel in such a manner during the winding up process that the reel exhibits bulges and valleys in accordance with the thick and thin points. Therefore, to obtain a cylindrical coil, the blown films produced by the blown film extrusion process are taken off in reverse or also rotatingly up to 360.degree. by means of the taking-off collapsing rollers so that the thick and thin spots are distributed on the reel like a coil of rope.
To control the measured thick and thin spots of the blown film, it is well-known to divide the die ring of the blown film die into heating sections or to provide heating elements, forming a ring around the extruded film bubble, by means of which the measured thick spots are heated and/or the measured thin spots are cooled so that the result is an adjustment and approximation of the average film thickness.
Since the thickness profile of the extruded film bubble is measured above the socalled frost line, thus in the region, which is distant from the heating sections or the heating elements, it is especially difficult to assign the measured thick and thin points correctly with respect to the position or the angle to the die ring or the heating elements in order to achieve the necessary heating effect on the measured thick or thin spots.
The DE-PS 29 47 293 discloses a process of the aforementioned kind, by means of which it is possible to perform by calculations or by suitable rotation and twisting of the measuring devices the requisite reversing of the measured thickness profile in order to assign the measured film sections correctly with respect to the position to the die ring sections, from which they were extruded. However, this calculated reversing is not always successful, since the twisting of the film is affected by the calibrating basket and the collapsing boards, which rotate with the takeoff rolls and to which the film adheres owing to the film's stickiness.